Those batteries won't work in the Serpent, you need to use bullet connectors due to the battery trays. You could try these Gens Ace batteries
http://www.giantcod.co.uk/gens-5000m...-p-406696.html
Although GiantCod show no stock at the mo they are moving warehouses, so hopefully will have more stock soon. There is also a Gens Ace store in the EU that you can buy from. But GiantCod are in the UK so ordering/delivery is easier, and there service is top notch.
Not sure about the ESC or motor as I run Tekin, but 2250kv may be a bit quick. My Tekin is 1900 kV and plenty quick enough. But I've never used the HW stuff so can't really say for sure.